250 PROJECTION MICROSCOPE [CH. IX 



at all times available and whenever available a heliostat is needed to 

 keep the light fixed in a given position, sunlight is practically dis- 

 carded and the electric light is employed for illumination. 



§ 399. Parts of a Projection Microscope. — These are named 

 in order, beginning with the electric lamp : See Figs. 192, 207-208. 



(1) An arc lamp with constant current and rheostat adjustable 

 from about 8 to 20 amperes. 



(2) Lamp condenser. 



(3) Water bath for removing the heat rays. 



(4) Special achromatic condenser for high powers. 



(5) Large square stage with opening 6 centimeters in diameter. 



(6) Mechanical stage with wide range of movement. 



(7) A very wide tube for receiving the objectives and projection 

 oculars. This tube to be connected with coarse and fine adjustment 

 for focusing. 



All of these parts should be independent and adjustable so that 

 any one of them cau be adjusted or removed without interfering with 

 the others. 



In accordance with the suggestion of Dr. Coplin all of the appa- 

 ratus, including the mountings of the objectives should be dead black 

 to avoid reflections. Reflections are so dazzling that the operator can- 

 not properly focus the image on the screen. 



§ 400. The Arc Lamp for the Projection Microscope. — This 

 should have the following characters: (a) The carbons should be 

 tilted so that the crater in the positive carbon is nearly vertical. It 

 then sends the maximum number of rays to the lamp condenser. 

 (/>) The lower carbon should be slightly in advance of the upper one 

 as shown in Fig. 204 so that the crater in the positive carbon is on the 

 front of that carbon (Figs. 204-205) that is it should be in position 

 to throw the light toward the condenser. 



(c) The lamp must be adjustable vertically and horizontally, and 

 the carbon holders must also be adjustable so that the carbons may be 

 put in line from side to side and front to back. One cannot get satis- 

 factory results unless all these adjustments are possible. 



Hand-feed and automatic lamps are both used for projection. The 

 consensus of opinion among experts is that the hand-feed lamp is bet- 

 ter for photography and for projection. That has been the experi- 

 ence of the writer also. 



S401. Starting the Lamp. — For starting the lamp with the 

 hand-feed form it is necessary to bring the carbons in contact and then 



